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Hydrous pyrolysis of model compounds
Org. Geochem. Vol. 14, No. 4, pp. 365-373, 1989 Printed in Great Britain. All rights reserved
0146-6380/89 $3.00+ 0.00 Copyright 1989 MaxwellPergamon Macmillanplc
Hydrous pyrolysis of model compounds J. W. SMITH1, B. D. BATTS2 and T. D. GILBERT 1'3 tCSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, N.S.W. 2113, 2School of Chemistry, Macquarie University, North Ryde, N.S.W. 2113 and 3Now at State Electricity Commission of Victoria, P.O. Box 195, Morwell, Vic. 3840, Australia (Received 15 September 1987; accepted 5 December 1988) Ab~raet--In pursuit of a method for the determination of the total n-alkyl content of petroleum source rocks, the reactions of model organic compounds containing a long-chain n-alkyl constituent with water have been studied in sealed tubes. At 330°C, the n-alkyl component of n-alcohols and n-ethers and the n-alcoholic components of esters are largely converted to, and released as, the corresponding n-alkanes. Some fragmentation of carbon chains accompanies this reaction. The conversion of n-alkanoic acids to n-alkanes is only partial, even at 350°C, and n-alkyl aromatics and n-alkyl hydroaromatics remain virtually unaffected under these conditions. Where significant n-alkane generation from model compounds is observed, additions of brown coal generally promote this process. In view of the stability of n-alkanoic acids and n-alkylated hydrocarbons observed under the conditions of hydrous pyrolysis used in this study, this technique is not entirely suitable for determining the total n-alkyl content of all source rocks nor, presumably, their full petroleum potential. Key words--hydrous pyrolysis, model compounds, thermal stability, petroleum potential
INTRODUCTION It has been suggested (Smith et al., 1987) that, since Australian terrestrially-sourced crudes are characterized by a high content of n-alkanes, the longer-chain, n-alkyl content of immature source rocks could be the basis of a useful guide to the petroleum potential (prospectivity) of such rocks. The n-alkyl chains in immature sediments may occur as free hydrocarbons or in a wide variety of hydrocarbon precursor forms such as acids, esters, alcohols, ethers, etc. Therefore, any technique selected for assessing petroleum potential is required to have the widest capability for determining n-alkyl groupings. A definitive method for this purpose has not been described. Hydrous pyrolysis (Lewan et al., 1979; Lewan, 1985) holds some promise in this regard because, under the experimental conditions described (heating sediments with excess water in closed vessels at 330°C for 3 days), n-alkane yields are dramatically increased (Brooks and Smith, 1969). However, the hydrous pyrolysis method remains essentially arbitrary in nature because a fundamental understanding of the prevailing chemical reactions and physical processes occurring during hydrous pyrolysis and their degree of completion is lacking. In particular, as discussed in the next section, the role of parent carbonaceous material in promoting these reactions needs to be clarified. The objectives of the present work are (a) to provide quantitative fundamental information by investigating the reactions of a range of model hydrocarbons and oxygenated compounds, all with an n-alkyl component, under hydrous pyrolytic conditions, (b) to determine the role of brown coal and a
range of additives in promoting these reactions, and (c), on the evidence collected, to assess hydrous pyrolysis as a method for estimating the n-alkyl content of sediments as a guide to their petroleum potential.
PREVIOUS CHEMICAL STUDIES After hydrous pyrolysis in the presence of carbonaceous materials, free n-alkanes remain unaltered, n-alcohols are directly reduced to the corresponding n-alkanes and n-acids decarboxylate (Brooks and Smith, 1969; Hoering, 1984). However, the extent of such reactions has not been determined. Brooks and Smith (1969) also showed that, on heating cetyl palmitate with water alone at 330°C, l-hexadecene was generated from the alcoholic moiety whereas, when an equal quantity of solvent extract from the coal was included in the reaction mixture, the major product was the corresponding aikane. The conclusions reached included "The source of the hydrogen which effected the hydrogenation of the alkane is not known; it may have been partly derived from the conversion of cyclic terpenoid to aromatic ring structures". From the more recent investigation (Hoering, 1984), it is evident that the hydrogen produced and consumed during hydrous pyrolysis is derived by the thermal alteration of the kerogen (solvent extracted Messel shale), although the precise role of water is still not clear. Hoering (1984) also demonstrated (for a range of compounds including normal, iso and ante-iso alkanes, isoprenoid hydrocarbons, steranes and triterpanes) that deuterated products always
365
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J.W. SMITHet al.
result when bond cleavage occurs during hydrous pyrolysis in the presence of D20. These deuterated products are of particular interest because, not only may their distribution provide a guide to the position of the original linkage of the product to the kerogen but also their presence, especially in product nalkanes, confirms that they do not represent original hydrocarbons physically entrapped within the kerogen. A factor of significance in determining the yield and distribution of alkanes in oils obtained by the hydrous pyrolysis of coals and source rocks is the thermal stability of the carbon skeletal structures of their precursors. In this respect, in the evaluation of data it is essential to know, for example, whether long-chain alkyl aromatics are cleaved and thus contribute to the liquid alkane product, and the degree to which branched and cyclic alkanes may be aromatized or converted to gas. Some insight into these questions has been provided by the results of solvent extraction, controlled hydrogenation and hydrous pyrolysis studies of a range of Victorian brown coal lithotypes (Rigby et al., 1986a). In that work, the n-alkanes appeared to be generated from their precursors over a range of sediment maturity, expressed in terms of vitrinite reflectance, a feature which might well be related to the chemical nature of the n-alkane precursor (kerogen type) or to its mode of bonding to the parent coal "molecule". At present, some divergence of opinion exists on the level of sediment maturity (vitrinite reflectance), at which sediments pass through the oil window (Price and Barker, 1985). The experimental findings of Rigby et al. (1986a) possibly provide, in geochemical terms, an explanation for apparent deviations in sediment maturity with regard to oil and/or gas generation. In distantly related studies, the thermal stabilities of n-alkanoic acids, their release from kerogen and the subsequent generation of their n-alkane products have been investigated in some detail (Jurg and Eisma, 1964; Kawamura and Ishiwatari, 1985; Kawamura et aL, 1986a, b). The temperatures, periods of heating and conditions described by these authors vary considerably and generally differ from those employed in hydrous pyrolysis, and therefore comparison and evaluation of resultant data are difficult. However, the general conclusion, that the high relative thermal stability of carboxylic acids under pyrolysis conditions is further enhanced by the presence of water, is clearly of significance and a factor to be taken into consideration in the interpretation of the results reported here. EXPERIMENTAL
Model compound selection A wide, but not exhaustive, range of model compounds of the types likely to occur in immature sediments and which also contained a long-chain
Table 1. Model compounds studied n-Octadeeanol n-Hexadecanoic acid n-Hexadecyl n-hexadecanoate (spermaceti)' Di-n-pentadecyl ketone (palmitone) Di-n-hexadecyl etherb Phenyl n-octadecanoate n-Octadecyl benzoate n-Octadecyl phenyl ether n-Hexadecyl phenyl ketone n-Octadeeyl benzene n-Octadecyl cyclohexane Octadecyl benzenec Octadecyl cyclohexanec Octadecyl toluenec 'Natural product: ratio of n-C~4:n-C~6:n-C~8 in alkyl chains: 18:74:8. bNot a single compound; ratio of n-El4: n-El5:n-Ci6 :n-Ci7: n -Ci8 in alkyl chains: 8.3:3.6:81.5:0.9:5.5 CMixed branched isomers. Prepared from n-octadecanol and corresponding hydrocarbons by method of Rigby et al. (1986b).
n-alkyl component was selected for testing under the conditions of hydrous pyrolysis. Included in this study were n-carboxylic acids, n-alcohols and a range of pure n-alkyl esters, n-ketones and n-ethers and their corresponding aromatic counterparts. In addition to these oxygenated materials, several n-alkyl and branched alkyl substituted aromatic and hydroaromatic hydrocarbons were also investigated under hydrous pyrolysis conditions. Pure compounds of the types required were not always available and several of the compounds used, although true to chemical type, consisted of more than one compound. For example, the nominal di-nhexadecylether, which was synthesized from stock cetyl alcohol, comprised a range of di-n-aikyl ethers ranging from the n-Cl6/n-Cl4 to the n-Cl6/n-Cls dervatives; spermaceti, the natural product ester, similarly comprised a series of compounds. A list of the model compounds investigated and their descriptions is given in Table 1. Experimental procedures The standard procedure was to transfer 20-50 mg of the model compound, 2.0-5.0 mg of the C20 or C36 n-alkane (for quantification) and 2 ml of water to a stainless steel reaction vessel of 10 ml capacity. This quantity of water ensured the presence of liquid water at all stages of the experiments. In many experiments the reaction vessels were closed in air at this stage but in others different weighed quantities of the mediumlight lithotype of Morwell brown coal (maximum weight 2 g) were added before tube closure. On a dry, mineral-free basis this coal contained C, 71.5%; H, 5.2%; N, 0.7%; S, 0.6%; and O, 22.1%. The ash content of the dry coal was 3.4% with Ca, 0.79%; S, 0.63%; Fe, 0.31%; and Mg 0.26% being the major components. In a series of experiments in which palmitic acid, alone and in mixtures with brown coal, was used, additions of bentonite, calcium hydroxide, copper chromite, basic copper carbonate and metallic zinc and tin were made to the reactants. In specific experiments with stearyl benzoate and phenyl
Hydrous pyrolysis of model compounds stearate, the reactors were pressurized with hydrogen to 6.9-7.4 MPa before closure. In general the reactors were heated at 330°C for 3 days in a stationary mode (rocking having been shown to be without significant effect) but in two experiments with palmitic acid and the n-alkyl substituted benzene a temperature of 350°C was used. After cooling, the contents of the reactors were washed out with methylene chloride following acidification with hydrochloric acid in experiments in which calcium hydroxide, carbonates or metallic powders had been added. After filtration from solid' residues, the nature and distribution of the methylene chloride-soluble products were determined by chromatographic separation on a SGE 50 m x 0.2 mm I.D., SE30 WCOT silica column. The column temperature was held at 10°C for 2rain, programmed at 4°C/rain to 290°C and then held at that temperature. Contributions from the coal to the hydrous pyrolysis products were determined from experiments in which weighed quantities of coal and water were reacted alone. Analysis of these products resulted in reproducible, but complex, chromatograms with a high concentration of larger unresolved features at retention times in the range of the Cs to Cmsn-alkanes. Therefore, where ratios of added coal to model compound were in excess of 20, some difficulty was experienced in determining with precision the quantities of the shorter chain n-alkanes (< n-C16) generated from the model compound. This problem became more pronounced as the quantity of n-alkanes generated decreased. However, even at the highest ratios of coal to model compound used, concentrations of individual n-alkanes (>n-C10) exceeding 2% by weight of the original added model
S0
O
lsc E ~C
"~ X
i 10
0
A[ky[ aromatic J / A l k y l hydroaromatic { no conversion Alkyl aryl ketone J 2~0 fi i i ~ 30 /-0 SO 6O RoUo of added coal to model Compound
. 70
8O
Fig. 1. C o n v e r s i o n o f m o d e l c o m p o u n d s to n - a l k a n e s .
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compound were dearly distinguishable above the chromatographic "envelope" of largely unresolved peaks due to the coal. The measurement of signals from these shorter chain n-alkanes above the "background" was unreliable. Therefore, in the tabulation of results from experiments in which the ratio of coal to model compound exceeded 20, n-alkanes in concentrations of less than 2% by weight were not included in calculations and are not reported. The overall extent of reaction, but not the distribution of product n-alkanes, was also calculated by the change in the ratio of added model compound and n-alkane "marker" which accompanied reaction. When phenol occurred in reaction products, its volatility and solubility in water made the precise measurement of concentration difficult. Where required, a precise identification of individual products was made using a Finnigan 4023 Quadrupole mass spectrometer system. Products were identified using either mass spectral library (NBS/EPA/NIH) comparison or by interpretation of fragmentation patterns in the mass spectrum.
RESULTS
Hydrous pyrolysis at 330°C with addition of coal The general data in terms of the conversion of pure n-alkyl compounds to n-alkanes containing more than l0 carbon atoms are summarized in Fig. 1. Details of the reactions and products of all compounds studied are given below. n-Octadecanol. The reactions of the n-alcohol are of particular interest because, under the wide range of conditions used, this was one of the few classes of compound in which complete conversion to nalkanes was observed. In the absence of coal, only 20.7% of the alcohol was converted to products, of which Cls unsaturated hydrocarbons and the n-Cl~ alkane were the major components together with minor n-octadecanaldehyde (Table 2, Figs 1 and 2). With increasing additions of coal, the unsaturated hydrocarbons and the aldehyde were no longer observed in the products and increasing conversions of the n-alcohol to n-alkanes occurred (Table 2, Fig. 1). n-AIkane generation was also accompanied by considerable C-C bond breakage within the n-alkyl chain, although the major products at high degrees of conversion continued to be the n-el7 and n-C18 alkanes. n-Hexadecanoic acid. As shown in Fig. 1 and Table 3, the n-acid was more resistant to conversion to n-alkanes than the n-octadecanol. Even when coal was added to the n-acid in a ratio of 60:1, less than 50% percent conversion of the n-acid to n-alkanes was obtained by hydrous pyrolysis. The resultant n-alkanes were dominated by the n-Cls member and, as was also observed in the corresponding experiments with the n-alcohol, considerable fracturing of the n-alkyl chain occurred.
J. W. SMn'Het al.
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Table 2. Major products from hydrous pyrolysisof n-octadecanolat 330°C for 3 days
Ratio coal: n -octadecanol 0 4 20 32
Cn 0.0 0.6 1.6 2.7
Ci2 0.0 0.9 2.6 3.7
n-Alkancs (%) Ci3 CI4 Cl5 CI6 (~17 0.0 <0.1 0.2 0.3 6.9 1.1 1.4 2.5 2.3 8.1 3.9 4.7 8.4 7.3 18.7 6.4 6.2 10.6 9.2 22.8
Spermaceti. This natural product consisted largely of n-hexadecyi n-hexadecanoate but more detailed analysis showed the total n-alkyl component to be comprised of approximately 74%, Cm6; 18%; C~4; and 8% C~s n-alkyl chains. In the absence of coal the ester was totally converted to a highly complex mixture of
Ui <
9 8
d Fig. 2. Gas chromatogram of products from hydrous pyrolysis of n-octadecanoi at 330°C for 3 days.
C18 1.6 I1.0 26.1 29.0
CIs n-C~s n-Cis unsaturated aldehyde alcohol 8.4 3.2 79.3 10.4 0 61.8 0 0 26.7 0 0 9.4
Conversionof n-alcohol to n-alkanes (%) 9.1 27.9 73.3 90.6
products in which mixed C~4, C~6 and C~s alkenes, and the n-C~4, n-Cl6 and n-C~s acids and alcohols predominated. No attempt was made to resolve this product fully. However in the presence of added coal, as shown in Table 4, the products from hydrous pyrolysis of the ester were alkene-free and fully resolvable into n-acids, n-alkanes and n-alcohols, with the n-C13, n-Ct5 and n-Cl6 members present in the highest concentrations in the n-alkane fraction. In these experiments the concentration of n-acids in the products always greatly exceeded that of the nalcohols although, with increasing concentrations of added coal, total conversions of ester to n-alkanes increased (Fig. l) and concentrations of both n-acids and n-alcohols decreased. Di-n-pentadecyl ketone. As shown in Table 5, the C14 and C~5 n-alkanes were always most prominent in the reaction products from this ketone, although the relative concentrations of the shorter chain n-alkanes increased with increasing additions of coal to the reactants and conversions of the ketone to n-alkanes (Fig. 1). n-Hexadecanoic acid was the other major reaction product when the ratio of added coal to ketone was small. Untriacontane (n-C3~) was an interesting minor product in all experiments. Di-n-hexadecyl ether. This was not a pure compound. The n-alkyl components comprised 8.3%, Cl4; 3.6%, Cls; 81.5%, Cl6; 0.9%, Cry and 5.5%, Cls n-alkyl chains. Conversion of this ether to the corresponding n-alkanes occurred quite readily when the ratio of coal to ether was small ( < 5: l) and even more so at extended reaction times (Table 6). Hexadecanol was a by-product, or intermediate, in such reactions
Table 3. Products from hydrouspyrolysisof n-hexadecanoicacid at 330°Cfor 3 days Conversion of n-acid Ratio n-Alkanes (%) Acid to n-alkanes coal:acid Ci0 Cu Cj2 Ci3 Ci, Ci5 Ci6 (%) (%) 0' 0.0 0.0 0.2 0.9 0.7 2.1 0.0 96.1 3.9 5 0.0 0.0 0.4 0.7 0.7 4.6 0.0 93.5 6.5 20 1.2 1.0 1.8 3.1 2.6 12.7 0.6 77.0 23.0 60 0.0 3.2 4.0 7.9 7.1 23.8 2.4 51.6 48.4 'Very minor quantities of alkenes observed. Table 4. Products from hydrous pyrolysisof spermaceti at 330°C for 3 days Conversion of ester to
Ratio coal:ester
20 49
C12 2.7 3.4
Ct) 6.9 10.l
n-Alkanes (%) Cj, C~s Cj6 0.5 11.7 13.0 7.3 14.2 14.5
Cl7 1.8 4.8
C1s 4.0 5.0
n-Acids (%) Ci4 Cl6 Cis 18.3 26.4 4.0 12.9 18.7 3.2
n-Alcohols (%) C)6 C1s 8.8 1.7 5.2 0.7
n-alkanes (%) 41 59
369
Hydrous pyrolysis of model compounds Table 5. Major products from hydrous pyrolysis of di-n-pentadecyl ketone (palmitone) at 330°C for 3 days Conversion of ketone to Ratio n-Alkanes (%) a-Acid (%) Paimimn¢ n-alkane coal:ketone C10 Cii Ci2 Ct3 C~( Cts Cl6 C3~ Cle (%) (%) 0 1.4 3.0 5.6 10.0 15.7 17.8 3.0 0.7 12.6 30.1 57.3 2 1.6 4.7 7.0 11.2 15.3 18.0 3.0 0.3 13.6 25.3 61.1 11 4.8 6.4 9.0 14.6 16.6 17.7 3.1 0.4 8.0 19.4 72.6 32 7.7 7.1 8.6 15.2 18.4 18.7 3.9 0.4 2.8 17.1 80.1 From comparison with added n-C22"marker", when ratio of coal to ketone used was 55, 3% of the palmitone remained unaltered and 3% as the n-C~6acid. Table 6. Major products from hydrous pyrolysis of di-n-alkyl ether at 330°C Duration of Ratio heating n-Alkanes (%) coal:ether (days) Clo Cii Ci2 CI3 Ci4 Ci5 CI6 0 3 1.6 5.7 15.4 5 3 0.4 0.6 1.0 2.5 4.5 8.8 29.0 2.5 5.5 - - 1.2 1.9 4.3 7.7 16.9 45.8 From comparison with the added n-C;2 "marker", when the ratio of coal to (Fig. 3). U n d e r conditions in which the ratio of added coal to ether was 50, the complexity of the reaction products prohibited a simple assessment of the product chromatograms. However, the virtual disappearance o f the ethers relative to the added n-C22 alkane " m a r k e r " demonstrated the complete decomposition of the ether under these conditions. Phenyl stearate. When pyrolysed alone, this ester was completely converted to the normal hydrolysis products, phenol and octadecanoic acid, together with 3.4% of the n-Cl7 alkane. When coal was added to the ester in the ratio 16:1 the hydrolysis products comprised almost 70% n-alkanes with the following distribution, C~2, 1.5%; Cm3, 3.7%; C~4, 5.4%; Cm5 10.1%; Clt, 8.8%; C17, 35.2% and Cls, 2.3%. The other products were phenol and octadecanoic acid.
Ci7 CIg
n-C~6 alcohol
Ether
Conversion of ether to n-alkancs
(%)
(%)
(%)
2.0 4.1 -71.0 29.0 3.7 3.6 8.7 37.2 54.1 4.3 6.5 3.7 7.6 88.7 ether used was 50, < I% of the ether remained unchanged.
Stearyl benzoate. In the absence of coal this ester was completely converted to a wide range of products, including benzoic acid, 11.5%; benzene, 13.3% and the n-C17 alkane, 3.3%. The major product was an unresolved mixture of the CI7 and Cms alkenes together with a small proportion of the n-C~8 alkane. When coal was added to this ester in the ratio o f 50:1, apart from benzene (14.4%) the products o f hydrous pyrolysis were n-alkanes with the following distribution Cl0, 1.5%; Cll, 1.3%; Ci2, 3.3%; Cj3, 6.0%; Cl(, 6.1%; Cls, 9.3%; Clt, 9.4%; ClT, 16.2% and Cls, 32.4%. n-Octadecyl phenyl ether. As shown in Table 7 the extent of conversion of the ether to n-alkanes was strongly dependent on the ratio o f added coal to ether. U n d e r all conditions used, n-octadecane was
"r 0
I o (.3 i
.1o QO
o
L
Produc~n- al kanes
Residual di-_n -ether
Fig. 3. (3as chromatogram of products from hydrous pyrolysis of the di-n-ether in the presence of coal (coal:ether ratio by weight = 5) at 330°C for 3 days.
J.W. SMITHet al.
370
Table 7. Major products from hydrous pyrolysis of n-octadecyl phenyl ether at 330°C for 3 days n-Alkanes (%) Ratio coal:ketone 2 6 20 53
CI4
Cts
CI~
Ci7
0.5 0.5
1.0 1.9
1.1 2.1
--3.4 4.3 6.3 10.9
Cis 5.0 9.5 14.8 65.6
the major alkane generated. In addition to phenol, n-octadecanol and CIs alkenes were significant reaction products, particularly when coal:ether ratios were small. Even when this ratio was increased to 53, the persistence of 9.2% of this ether in the reaction products testifies to the greater stability of the n-alkyl aryl ether relative to its di-n-alkyl counterpart. n-Hexadecyl phenyl ketone. This compound was very stable. In the absence of coal no reaction occurred. When a ratio of coal to ketone of 9 was used, 12% of the ketone was directly reduced to bexadecyl benzene and conversion to this compound was increased to 39% when a coal to ketone ratio of 52 was employed. Hydrocarbons (including n-octadecyl benzene, noctadecyl cyclohexane, and branched octadecyl isomers of benzene, cyclohexane and toluene). In the absence of coal and in reactions in which the ratio of added coal to model compound ranged from 5 to 98, no products indicating the cleavage of the side chain from the parent molecule were observed. Alkyl benzenes were reduced to the corresponding cyclohexane derivatives but the substituted toluenes remained unaffected.
Hydrous pyrolysis at 350°C with addition of coal n-Hexadecanoic acid. A ratio of added coal to n-acid of 15 was used in a single experiment at this temperature. Under these conditions 15% of the acid was converted to n-alkanes, a result not significantly different from that obtained by hydrous pyrolysis at 330°C (Fig. 1, Table 3). n-Octadecyl benzene. In another single experiment with an added coal to model compound ratio of 32, only 4% of the n-alkyl benzene was cleaved to generate mainly n-heptadecane with a little bexadecane. Toluene was also observed in the reaction products. Hydrous pyrolysis at 330°C with addition of molecular hydrogen n-Octadecanol. The reaction mixture, with a ratio of added coal to the n-alcohol of 4, was pressurized to 6.9-7.4 MPa with molecular hydrogen before closure of the vessel and heating to 330°C. The reaction products consisted of octadecane, 11.5%; heptadecane, 0.6%; hexadecane, 0.3%; mixed octadecenes, 2.1%; and unchanged alcohol 85.6%. Phenyl stearate. Hydrous pyrolysis of this ester in the absence of coal at a pressure of 6.9-7.4 MPa of molecular hydrogen resulted in complete hydrolysis
Alkenes Cis (%) 6.5 11.0 5.9 0.0
Alcohols n-Cls (%)
Phenol (%)
Ether (%)
2.8 4.7 3.9 4.0
8.0 7.8 13.0 10.3
74.3 60.2 51.6 9.2
to yield octadecanoic acid and phenol. No n-alkanes were observed in the reaction products.
Experiments with miscellaneous additives n-Hexadecanoic acid. In attempts to promote decarboxylation of the acid or direct reduction to the n-alkane, a range of additives was tested, including alumina, calcium hydroxide, copper chromite, basic copper carbonate, zinc and tin powders and various combinations of these additives together with coal. Temperatures in the range 275-330°C were used. No significant increases in n-alkane generation compared to those obtained by additions of coal alone were observed. n-Octadecanol. Similar attempts to improve the conversion of the alcohol to alkanes by the use of the additives listed above were also unsuccessful. DISCUSSION
From the data presented above on the hydrous pyrolysis of model compounds it is evident that the degree of conversion of n-alkyl components to nalkanes is dependent on (1) the chemical mode of linkage of the n-alkyl groups in the parent model compound and (2) the ratio of coal to model compounds employed in hydrous pyrolysis.
Relationship between chemical mode of attachment of n-alkyl components and ease of conversion to nalkanes To simplify discussion, the model compounds have been classified into four groups based on the ease with which conversion to n-alkanes was obtained. These groups--complete, moderate, difficult, resist a n t - a r e defined in Table 8 on the basis of the degree of conversion measured, or to be expected, when a ratio of coal to model compound of ,,, 80 is used in hydrous pyrolysis. As mentioned previously, at such large ratios of coal to model compound, a real difficulty exists in distinguishing the precise origins of the shorter chain n-alkanes in reaction products and in determining the extent of chain-shortening reactions. Therefore values given for conversion under such conditions may only be regarded as guides to the actual situation. Two factors related to molecular structure appear to be critical in determining the degree of conversion undergone by the model compounds studied: (1) the inherent thermal stability of the molecule and (2) the tendency for the molecule to form more stable prod-
Hydrous pyrolysis of model compounds ucts by cleavage or rearrangement under thermal stress. The model compounds classified as "complete" in Table 8 show none of these adverse characteristics. The n-alcohol, when sufficient coal is present, appears to transform via the alkene and the aldehyde to the corresponding n-alkane. Therefore the formation of small quantities of n-alcohols during the hydrous pyrolysis of the di-n-ether and the n-alkyl aryl ether is not regarded as a barrier to their total conversion. Similarly the alcoholic n-alkyl component of the n-octadecyl benzoate is rapidly freed by hydrous pyrolysis and proceeds along the same alkene pathway as the product n-alkanes. Benzoic acid, the other hydrolysis product, underwent partial decarboxylation to benzene. On precisely the same type of evidence, the di-nester was classified as moderate. In this case, following hydrolysis, the alcoholic portion of the ester was again free for conversion to n-alkanes, but the carboxylic acid fraction, now comprising some 50% of the total n-alkyl content of the original ester, was particularly resistant to further conversion by decarboxylation (Tables 3 and 4). On hydrous pyrolysis of the di-n-pentadecyl ketone, hexadecanoic acid was formed to a small degree by rearrangement of the parent molecule (Table 5). Therefore this ketone is classified as "moderate" by the criteria applied. Carboxylic acids and aryi esters of n-acids both automatically qualify as "difficult" in Table 8 because in esters of this type the total n-alkyl content is released as the n-acid on hydrous pyrolysis. Acids are partially resistant to decarboxylation, the major pathway for n-alkane formation, as shown in Table 3 and Fig. 1. The stability of acids under these conditions is in general agreement with the findings of Jurg and Eisma (1964), Kawamura and Ishiwatari (1985) and Kawamura et al. (1986a, b) discussed earlier. n-Alkyl aromatic and hydroaromatic compounds, and those compounds which are converted to such structures on hydrous pyrolysis, e.g. n-alkyl aryl ketones, were totally resistant to reaction. Clearly carbon-carbon bond cleavage in parent molecules of the types examined is not accomplished by hydrous
371
pyrolysis; n-alkanes added as "markers" exhibit a similar thermal stability. Nevertheless, the data presented in Tables 2-7 demonstrate unequivocally that when n-alkanes are generated by other mechanisms, e.g. by decarboxylation or by dehydration and alkene formation, significant cleavage of the generated carbon chains into smaller units occurs. Mechanisms relating to n-alkane generating processes in hydrous pyrolysis have been described by Hoering (1984).
The role of brown coal in promoting hydrous pyrolytic reactions From earlier work (Brooks and Smith, 1969; Hoering, 1984) and the bulk of the experimental data reported here, it is evident that the presence of carbonaceous material is essential if the generation of n-alkanes from the n-alkyl components of organic molecules is to proceed to a significant extent via hydrous pyrolysis in laboratory experiments and presumably in sedimentary environments. This role cannot be adequately described as catalytic since the extent of model compound conversion is closely related to the quantity of the brown coal added and, to achieve significant conversion, ratios of coal to model compounds >25 are commonly required. Moreover, from the data presented here, it is evident that the brown coal fills several roles. For example from Table 3, the generation of n-octadecane as the major hydrous pyrolysis product of n-octadecanol and the disappearance of the unsaturated Cls hydrocarbons in these products as the quantity of coal added is increased, suggests that a major role of the coal is as a hydrogen donor. On the other hand it is obvious from Table 4 that generation of n-alkanes from n-acids proceeds via a decarboxylation process promoted by the presence of the active surface of coal. Information on the nature of the hydrogeneration mechanisms prevailing during hydrous pyrolysis was provided by those experiments using n-octadecanol and phenyl stearate in which the autoclave was pressurized with molecular hydrogen to 6.9-7.4 MPa prior to hydrous pyrolysis. When using a coal to model compound ratio of 4:1, 27.9% of the n-
Table 8. Relative convertibilities of n-alkyl containing model compounds to n-alkanes by hydrous pyrolysis
Classification Complete
Moderate Difficult Resistant
Model compounds n-Alcohols Di-n-ethers n-Alkyl aryl ethers n-Esters of aryl acids Di-n-esters Di-n-ketones n -Acids Aryl esters of n-acids Aryl-alkyl ketone n-Alkyl aromatics and hydroaromatics
Conversion of of n-alkyl component to n-alkanes (%)
n-Alkyl compounds resistant to conversion to n-alkanes
> 90% > 70%
Minor n-acids
< 70% < 70% 0 0
Parent n-acid Major n-acids n-Alkyl aromatic Parent hydrocarbons and hydroaromatic derivative
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J.W. SMITHet al.
octadecanol was converted to n-alkanes by normal hydrous pyrolysis (Table 2), however, when the system was pressurized with molecular hydrogen as described, this conversion was reduced to 13.6%. Rather than promoting reduction, the molecular hydrogen appeared to behave as an inert diluent gas. A similar effect was seen when molecular hydrogen was introduced into experiments in which phenyl stearate underwent hydrous pyrolysis in the absence of the coal. In these experiments, whether hydrogen was added or not, complete hydrolysis of the ester to phenol and octadecanoic acid occurred. However, in the absence of added hydrogen, the reaction products also contained 3.40 heptadecane. Where the reactor had been pressurized with hydrogen, n-alkanes were not detected in the products. On this evidence, one role of the brown coal appears to be as a source of exchangeable active hydrogen, although the availability of such species for the reduction of model compounds is relatively small, in view of the high ratio of coal to model compound required to effect reduction. The role of the brown coal in promoting decarboxylation remains obscure. The quantity and composition of the inorganic and mineral matter associated with the brown coal and the ineffectiveness of additions of a range of inorganic materials in increasing decarboxylation provide little support for an inorganic decarboxylation mechanism. Another unknown factor in hydrous pyrolysis is the effect of the nature and composition of the carbonaceous material on conversion reactions. To date, although the hydrous pyrolysis products from several shales and coals have been analysed and evaluated (Brooks and Smith, 1969; Winters et al., 1983), only two materials, the medium-light brown coal lithotype used in the present study and a solvent extracted Messel shale (Hoering, 1984), have been used in experiments specifically designed to define the role of the carbonaceous additive in hydrous pyrolysis. It does not seem unlikely that the capacity of the carbonaceous material to generate active, reductive species, and the composition of associated mineral matter play a significant role in determining the degree of conversion obtained and the nature of the reaction products. Use o f hydrous pyrolysis in assessment o f long-chain n-alkyl contents o f sediments
The high degree of dependence of hydrous pyrolytic reactions on the presence and concentration of carbonaceous materials, the difficulties encountered in converting n-acids to n-alkanes and the complete resistance of n-alkyl aromatic and hydroaromatic hydrocarbons to any change suggest that caution should be observed in the assessment of results. From the evidence present, the results obtained for nalkane contents and distributions in such rocks are likely to be most in doubt in sediments in which (a) free n-carboxylic acids or their metal salts occur,
(b) carboxylic acids are released on hydrolysis and (c) n-alkyl aromatic and hydroaromatic compounds constitute a major portion of the total n-alkyl content. Monthioux et al. (1985) reported that many aspects of the behaviour of Type III organic matter during natural evolution could be reproduced satisfactorily by pyrolysis in a confined system. Moreover, hydrous pyrolysis appears to simulate sedimentary maturational processes when hydrocarbon products are compared with related crudes (Brooks and Smith, 1969; Winters et al., 1983). However difficulties arise when attempts are made to relate the changes in vitrinite reflectance that accompany hydrous pyrolysis to specific chemical reactions. For example, it appears to be well established within the coal rank series that the content of carboxylic groups in coals with carbon contents greater than 80% (Blom, 1960) or with vitrinite reflectance values in excess of 0.7*/0 (Evans et al., 1984) is negligible. However, in experiments reported here, at temperatures of 350°C, decarboxylation of hexadecanoic acid only proceeded to 15% although the vitrinite reflectance of the brown coal included in the reactants had increased from 0.2 to 1.3%. CONCLUSIONS Although the product oils from hydrous pyrolysis may in many respects resemble crudes, the data presented here indicate that the conversion to hydrocarbons of some important n-alkane precursors may be incomplete. Factors that require consideration in the assessment of hydrous pyrolysis as a method for the determination of the long-chain n-alkyl contents of source rocks and hence as a guide to their petroleum potential include: (a) the stability of n-alkanoic acids and nalkylated hydrocarbons under the hydrous pyrolytic conditions investigated (b) the major, but little-understood, role of brown coal (kerogen) in promoting n-alkane generation and release; (c) possible variations in this role with kerogen type; and (d) difficulties in relating specific chemical changes observed with increases in vitrinite reflectance. Acknowledgements---Grateful acknowledgments are made to the State Electricity Commission of Victoria for provision and analysis of the brown coal. REFERENCES
Blom L. (1960) Analytical Methods in Coal Chemistry. Luijk, Eindhoven. Brooks J. D. and Smith J. W. (1969) The diagensis of plant lipids during the formation of coal, petroleum and natural gas--II. Coalification and the formation of oil and gas in
Hydrous pyrolysis of model compounds the Gippsland Basin. Geochim. Cosmochim. Acta 33, 1183-1194. Evans E. J., Batts B. D. and Smith J. W. (1984) Determination of the hydrocarbon prospoaivity of sediments by hydrogenation. APEA J. 24, 222-229. Hoofing T. C. (1984) Thermal reactions of kerogen with added water, heavy water and pure organic substances. Org. Geochem. S, 267-278. Jurg J. W. and Eisma E. (1964) Petroleum hydrocarbons: generation from fatty acid. Science, N.Y. 144, 1451-1452. Kawamura K. and Ishiwatari R. (1985) B¢haviour of lipid compounds on laboratory heating of a recent sediment. Geochem. J. 19, 113-126. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986a) Volatile organic acids generated from kerogen during laboratory heating. Geochem. J. 20, 51-59. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986b) Long-chain carboxylic acids in pyrolysates of Green River kerogen. Org. Geochem. 10, 1059-1065. Lcwan M. D. (1985) Evaluation of petroleum generation by hydrous pyrolysis experimentation. Phil. Trans. R. Soc. Lond..4,315, 123-134. Lewan M. D., Winters J. C. and McDonald J. H. (1979) Generation of oil-like pyrolyzates from organic-rich shales. Science, N.Y. 203, 897-899.
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Monthioux M., Landals P. and Monin J-C. (1985) Comparison between natural and artificial maturation series of humic coals from the Mahakam delta, Indonesia. Org. Geochem. 8, 275-292. Price L. C. and Barker C. E. (1985) Suppression of vitrinit¢ r¢fl¢ctanc¢ in amorphous rich kerogon--a major unrecognized problem. J. Pet. Geol. 8, 59-84. Rigby D., Gilbert T. D., Batts B. D. and Smith J. W. (1986a) The generation and release of hydrocarbons from Victorian brown coal lithotypes. In Second South-Eastern Australia Oil Exploration Symposium, Technical Papers Presented at PESA Symposium, 14-15 November 1985, Melbourne (Edited by Glenie R. C.), pp. 433-438. Pet. Explor. Soc. Australia, Melbourne. Righy D., Gilbert T. D. and Smith J. W. (1986b) The synthesis of alkyl aromatic hydrocarbons and its geochemical implications. Org. Geochem. 9, 255-264. Smith J. W., Batts B. D. and Gilbert T. D. (1987) A quest for a new parameter in petroleum exploration geochemistry. APEA J. 27, 98-105. Winters J. C., Williams J. A. and Lewan M. D. (1983) A laboratory study of petroleum generation by hydrous pyrolysis. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 524-533. Wiley, New York.
0146-6380/89 $3.00+ 0.00 Copyright 1989 MaxwellPergamon Macmillanplc
Hydrous pyrolysis of model compounds J. W. SMITH1, B. D. BATTS2 and T. D. GILBERT 1'3 tCSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, N.S.W. 2113, 2School of Chemistry, Macquarie University, North Ryde, N.S.W. 2113 and 3Now at State Electricity Commission of Victoria, P.O. Box 195, Morwell, Vic. 3840, Australia (Received 15 September 1987; accepted 5 December 1988) Ab~raet--In pursuit of a method for the determination of the total n-alkyl content of petroleum source rocks, the reactions of model organic compounds containing a long-chain n-alkyl constituent with water have been studied in sealed tubes. At 330°C, the n-alkyl component of n-alcohols and n-ethers and the n-alcoholic components of esters are largely converted to, and released as, the corresponding n-alkanes. Some fragmentation of carbon chains accompanies this reaction. The conversion of n-alkanoic acids to n-alkanes is only partial, even at 350°C, and n-alkyl aromatics and n-alkyl hydroaromatics remain virtually unaffected under these conditions. Where significant n-alkane generation from model compounds is observed, additions of brown coal generally promote this process. In view of the stability of n-alkanoic acids and n-alkylated hydrocarbons observed under the conditions of hydrous pyrolysis used in this study, this technique is not entirely suitable for determining the total n-alkyl content of all source rocks nor, presumably, their full petroleum potential. Key words--hydrous pyrolysis, model compounds, thermal stability, petroleum potential
INTRODUCTION It has been suggested (Smith et al., 1987) that, since Australian terrestrially-sourced crudes are characterized by a high content of n-alkanes, the longer-chain, n-alkyl content of immature source rocks could be the basis of a useful guide to the petroleum potential (prospectivity) of such rocks. The n-alkyl chains in immature sediments may occur as free hydrocarbons or in a wide variety of hydrocarbon precursor forms such as acids, esters, alcohols, ethers, etc. Therefore, any technique selected for assessing petroleum potential is required to have the widest capability for determining n-alkyl groupings. A definitive method for this purpose has not been described. Hydrous pyrolysis (Lewan et al., 1979; Lewan, 1985) holds some promise in this regard because, under the experimental conditions described (heating sediments with excess water in closed vessels at 330°C for 3 days), n-alkane yields are dramatically increased (Brooks and Smith, 1969). However, the hydrous pyrolysis method remains essentially arbitrary in nature because a fundamental understanding of the prevailing chemical reactions and physical processes occurring during hydrous pyrolysis and their degree of completion is lacking. In particular, as discussed in the next section, the role of parent carbonaceous material in promoting these reactions needs to be clarified. The objectives of the present work are (a) to provide quantitative fundamental information by investigating the reactions of a range of model hydrocarbons and oxygenated compounds, all with an n-alkyl component, under hydrous pyrolytic conditions, (b) to determine the role of brown coal and a
range of additives in promoting these reactions, and (c), on the evidence collected, to assess hydrous pyrolysis as a method for estimating the n-alkyl content of sediments as a guide to their petroleum potential.
PREVIOUS CHEMICAL STUDIES After hydrous pyrolysis in the presence of carbonaceous materials, free n-alkanes remain unaltered, n-alcohols are directly reduced to the corresponding n-alkanes and n-acids decarboxylate (Brooks and Smith, 1969; Hoering, 1984). However, the extent of such reactions has not been determined. Brooks and Smith (1969) also showed that, on heating cetyl palmitate with water alone at 330°C, l-hexadecene was generated from the alcoholic moiety whereas, when an equal quantity of solvent extract from the coal was included in the reaction mixture, the major product was the corresponding aikane. The conclusions reached included "The source of the hydrogen which effected the hydrogenation of the alkane is not known; it may have been partly derived from the conversion of cyclic terpenoid to aromatic ring structures". From the more recent investigation (Hoering, 1984), it is evident that the hydrogen produced and consumed during hydrous pyrolysis is derived by the thermal alteration of the kerogen (solvent extracted Messel shale), although the precise role of water is still not clear. Hoering (1984) also demonstrated (for a range of compounds including normal, iso and ante-iso alkanes, isoprenoid hydrocarbons, steranes and triterpanes) that deuterated products always
365
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J.W. SMITHet al.
result when bond cleavage occurs during hydrous pyrolysis in the presence of D20. These deuterated products are of particular interest because, not only may their distribution provide a guide to the position of the original linkage of the product to the kerogen but also their presence, especially in product nalkanes, confirms that they do not represent original hydrocarbons physically entrapped within the kerogen. A factor of significance in determining the yield and distribution of alkanes in oils obtained by the hydrous pyrolysis of coals and source rocks is the thermal stability of the carbon skeletal structures of their precursors. In this respect, in the evaluation of data it is essential to know, for example, whether long-chain alkyl aromatics are cleaved and thus contribute to the liquid alkane product, and the degree to which branched and cyclic alkanes may be aromatized or converted to gas. Some insight into these questions has been provided by the results of solvent extraction, controlled hydrogenation and hydrous pyrolysis studies of a range of Victorian brown coal lithotypes (Rigby et al., 1986a). In that work, the n-alkanes appeared to be generated from their precursors over a range of sediment maturity, expressed in terms of vitrinite reflectance, a feature which might well be related to the chemical nature of the n-alkane precursor (kerogen type) or to its mode of bonding to the parent coal "molecule". At present, some divergence of opinion exists on the level of sediment maturity (vitrinite reflectance), at which sediments pass through the oil window (Price and Barker, 1985). The experimental findings of Rigby et al. (1986a) possibly provide, in geochemical terms, an explanation for apparent deviations in sediment maturity with regard to oil and/or gas generation. In distantly related studies, the thermal stabilities of n-alkanoic acids, their release from kerogen and the subsequent generation of their n-alkane products have been investigated in some detail (Jurg and Eisma, 1964; Kawamura and Ishiwatari, 1985; Kawamura et aL, 1986a, b). The temperatures, periods of heating and conditions described by these authors vary considerably and generally differ from those employed in hydrous pyrolysis, and therefore comparison and evaluation of resultant data are difficult. However, the general conclusion, that the high relative thermal stability of carboxylic acids under pyrolysis conditions is further enhanced by the presence of water, is clearly of significance and a factor to be taken into consideration in the interpretation of the results reported here. EXPERIMENTAL
Model compound selection A wide, but not exhaustive, range of model compounds of the types likely to occur in immature sediments and which also contained a long-chain
Table 1. Model compounds studied n-Octadeeanol n-Hexadecanoic acid n-Hexadecyl n-hexadecanoate (spermaceti)' Di-n-pentadecyl ketone (palmitone) Di-n-hexadecyl etherb Phenyl n-octadecanoate n-Octadecyl benzoate n-Octadecyl phenyl ether n-Hexadecyl phenyl ketone n-Octadeeyl benzene n-Octadecyl cyclohexane Octadecyl benzenec Octadecyl cyclohexanec Octadecyl toluenec 'Natural product: ratio of n-C~4:n-C~6:n-C~8 in alkyl chains: 18:74:8. bNot a single compound; ratio of n-El4: n-El5:n-Ci6 :n-Ci7: n -Ci8 in alkyl chains: 8.3:3.6:81.5:0.9:5.5 CMixed branched isomers. Prepared from n-octadecanol and corresponding hydrocarbons by method of Rigby et al. (1986b).
n-alkyl component was selected for testing under the conditions of hydrous pyrolysis. Included in this study were n-carboxylic acids, n-alcohols and a range of pure n-alkyl esters, n-ketones and n-ethers and their corresponding aromatic counterparts. In addition to these oxygenated materials, several n-alkyl and branched alkyl substituted aromatic and hydroaromatic hydrocarbons were also investigated under hydrous pyrolysis conditions. Pure compounds of the types required were not always available and several of the compounds used, although true to chemical type, consisted of more than one compound. For example, the nominal di-nhexadecylether, which was synthesized from stock cetyl alcohol, comprised a range of di-n-aikyl ethers ranging from the n-Cl6/n-Cl4 to the n-Cl6/n-Cls dervatives; spermaceti, the natural product ester, similarly comprised a series of compounds. A list of the model compounds investigated and their descriptions is given in Table 1. Experimental procedures The standard procedure was to transfer 20-50 mg of the model compound, 2.0-5.0 mg of the C20 or C36 n-alkane (for quantification) and 2 ml of water to a stainless steel reaction vessel of 10 ml capacity. This quantity of water ensured the presence of liquid water at all stages of the experiments. In many experiments the reaction vessels were closed in air at this stage but in others different weighed quantities of the mediumlight lithotype of Morwell brown coal (maximum weight 2 g) were added before tube closure. On a dry, mineral-free basis this coal contained C, 71.5%; H, 5.2%; N, 0.7%; S, 0.6%; and O, 22.1%. The ash content of the dry coal was 3.4% with Ca, 0.79%; S, 0.63%; Fe, 0.31%; and Mg 0.26% being the major components. In a series of experiments in which palmitic acid, alone and in mixtures with brown coal, was used, additions of bentonite, calcium hydroxide, copper chromite, basic copper carbonate and metallic zinc and tin were made to the reactants. In specific experiments with stearyl benzoate and phenyl
Hydrous pyrolysis of model compounds stearate, the reactors were pressurized with hydrogen to 6.9-7.4 MPa before closure. In general the reactors were heated at 330°C for 3 days in a stationary mode (rocking having been shown to be without significant effect) but in two experiments with palmitic acid and the n-alkyl substituted benzene a temperature of 350°C was used. After cooling, the contents of the reactors were washed out with methylene chloride following acidification with hydrochloric acid in experiments in which calcium hydroxide, carbonates or metallic powders had been added. After filtration from solid' residues, the nature and distribution of the methylene chloride-soluble products were determined by chromatographic separation on a SGE 50 m x 0.2 mm I.D., SE30 WCOT silica column. The column temperature was held at 10°C for 2rain, programmed at 4°C/rain to 290°C and then held at that temperature. Contributions from the coal to the hydrous pyrolysis products were determined from experiments in which weighed quantities of coal and water were reacted alone. Analysis of these products resulted in reproducible, but complex, chromatograms with a high concentration of larger unresolved features at retention times in the range of the Cs to Cmsn-alkanes. Therefore, where ratios of added coal to model compound were in excess of 20, some difficulty was experienced in determining with precision the quantities of the shorter chain n-alkanes (< n-C16) generated from the model compound. This problem became more pronounced as the quantity of n-alkanes generated decreased. However, even at the highest ratios of coal to model compound used, concentrations of individual n-alkanes (>n-C10) exceeding 2% by weight of the original added model
S0
O
lsc E ~C
"~ X
i 10
0
A[ky[ aromatic J / A l k y l hydroaromatic { no conversion Alkyl aryl ketone J 2~0 fi i i ~ 30 /-0 SO 6O RoUo of added coal to model Compound
. 70
8O
Fig. 1. C o n v e r s i o n o f m o d e l c o m p o u n d s to n - a l k a n e s .
367
compound were dearly distinguishable above the chromatographic "envelope" of largely unresolved peaks due to the coal. The measurement of signals from these shorter chain n-alkanes above the "background" was unreliable. Therefore, in the tabulation of results from experiments in which the ratio of coal to model compound exceeded 20, n-alkanes in concentrations of less than 2% by weight were not included in calculations and are not reported. The overall extent of reaction, but not the distribution of product n-alkanes, was also calculated by the change in the ratio of added model compound and n-alkane "marker" which accompanied reaction. When phenol occurred in reaction products, its volatility and solubility in water made the precise measurement of concentration difficult. Where required, a precise identification of individual products was made using a Finnigan 4023 Quadrupole mass spectrometer system. Products were identified using either mass spectral library (NBS/EPA/NIH) comparison or by interpretation of fragmentation patterns in the mass spectrum.
RESULTS
Hydrous pyrolysis at 330°C with addition of coal The general data in terms of the conversion of pure n-alkyl compounds to n-alkanes containing more than l0 carbon atoms are summarized in Fig. 1. Details of the reactions and products of all compounds studied are given below. n-Octadecanol. The reactions of the n-alcohol are of particular interest because, under the wide range of conditions used, this was one of the few classes of compound in which complete conversion to nalkanes was observed. In the absence of coal, only 20.7% of the alcohol was converted to products, of which Cls unsaturated hydrocarbons and the n-Cl~ alkane were the major components together with minor n-octadecanaldehyde (Table 2, Figs 1 and 2). With increasing additions of coal, the unsaturated hydrocarbons and the aldehyde were no longer observed in the products and increasing conversions of the n-alcohol to n-alkanes occurred (Table 2, Fig. 1). n-AIkane generation was also accompanied by considerable C-C bond breakage within the n-alkyl chain, although the major products at high degrees of conversion continued to be the n-el7 and n-C18 alkanes. n-Hexadecanoic acid. As shown in Fig. 1 and Table 3, the n-acid was more resistant to conversion to n-alkanes than the n-octadecanol. Even when coal was added to the n-acid in a ratio of 60:1, less than 50% percent conversion of the n-acid to n-alkanes was obtained by hydrous pyrolysis. The resultant n-alkanes were dominated by the n-Cls member and, as was also observed in the corresponding experiments with the n-alcohol, considerable fracturing of the n-alkyl chain occurred.
J. W. SMn'Het al.
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Table 2. Major products from hydrous pyrolysisof n-octadecanolat 330°C for 3 days
Ratio coal: n -octadecanol 0 4 20 32
Cn 0.0 0.6 1.6 2.7
Ci2 0.0 0.9 2.6 3.7
n-Alkancs (%) Ci3 CI4 Cl5 CI6 (~17 0.0 <0.1 0.2 0.3 6.9 1.1 1.4 2.5 2.3 8.1 3.9 4.7 8.4 7.3 18.7 6.4 6.2 10.6 9.2 22.8
Spermaceti. This natural product consisted largely of n-hexadecyi n-hexadecanoate but more detailed analysis showed the total n-alkyl component to be comprised of approximately 74%, Cm6; 18%; C~4; and 8% C~s n-alkyl chains. In the absence of coal the ester was totally converted to a highly complex mixture of
Ui <
9 8
d Fig. 2. Gas chromatogram of products from hydrous pyrolysis of n-octadecanoi at 330°C for 3 days.
C18 1.6 I1.0 26.1 29.0
CIs n-C~s n-Cis unsaturated aldehyde alcohol 8.4 3.2 79.3 10.4 0 61.8 0 0 26.7 0 0 9.4
Conversionof n-alcohol to n-alkanes (%) 9.1 27.9 73.3 90.6
products in which mixed C~4, C~6 and C~s alkenes, and the n-C~4, n-Cl6 and n-C~s acids and alcohols predominated. No attempt was made to resolve this product fully. However in the presence of added coal, as shown in Table 4, the products from hydrous pyrolysis of the ester were alkene-free and fully resolvable into n-acids, n-alkanes and n-alcohols, with the n-C13, n-Ct5 and n-Cl6 members present in the highest concentrations in the n-alkane fraction. In these experiments the concentration of n-acids in the products always greatly exceeded that of the nalcohols although, with increasing concentrations of added coal, total conversions of ester to n-alkanes increased (Fig. l) and concentrations of both n-acids and n-alcohols decreased. Di-n-pentadecyl ketone. As shown in Table 5, the C14 and C~5 n-alkanes were always most prominent in the reaction products from this ketone, although the relative concentrations of the shorter chain n-alkanes increased with increasing additions of coal to the reactants and conversions of the ketone to n-alkanes (Fig. 1). n-Hexadecanoic acid was the other major reaction product when the ratio of added coal to ketone was small. Untriacontane (n-C3~) was an interesting minor product in all experiments. Di-n-hexadecyl ether. This was not a pure compound. The n-alkyl components comprised 8.3%, Cl4; 3.6%, Cls; 81.5%, Cl6; 0.9%, Cry and 5.5%, Cls n-alkyl chains. Conversion of this ether to the corresponding n-alkanes occurred quite readily when the ratio of coal to ether was small ( < 5: l) and even more so at extended reaction times (Table 6). Hexadecanol was a by-product, or intermediate, in such reactions
Table 3. Products from hydrouspyrolysisof n-hexadecanoicacid at 330°Cfor 3 days Conversion of n-acid Ratio n-Alkanes (%) Acid to n-alkanes coal:acid Ci0 Cu Cj2 Ci3 Ci, Ci5 Ci6 (%) (%) 0' 0.0 0.0 0.2 0.9 0.7 2.1 0.0 96.1 3.9 5 0.0 0.0 0.4 0.7 0.7 4.6 0.0 93.5 6.5 20 1.2 1.0 1.8 3.1 2.6 12.7 0.6 77.0 23.0 60 0.0 3.2 4.0 7.9 7.1 23.8 2.4 51.6 48.4 'Very minor quantities of alkenes observed. Table 4. Products from hydrous pyrolysisof spermaceti at 330°C for 3 days Conversion of ester to
Ratio coal:ester
20 49
C12 2.7 3.4
Ct) 6.9 10.l
n-Alkanes (%) Cj, C~s Cj6 0.5 11.7 13.0 7.3 14.2 14.5
Cl7 1.8 4.8
C1s 4.0 5.0
n-Acids (%) Ci4 Cl6 Cis 18.3 26.4 4.0 12.9 18.7 3.2
n-Alcohols (%) C)6 C1s 8.8 1.7 5.2 0.7
n-alkanes (%) 41 59
369
Hydrous pyrolysis of model compounds Table 5. Major products from hydrous pyrolysis of di-n-pentadecyl ketone (palmitone) at 330°C for 3 days Conversion of ketone to Ratio n-Alkanes (%) a-Acid (%) Paimimn¢ n-alkane coal:ketone C10 Cii Ci2 Ct3 C~( Cts Cl6 C3~ Cle (%) (%) 0 1.4 3.0 5.6 10.0 15.7 17.8 3.0 0.7 12.6 30.1 57.3 2 1.6 4.7 7.0 11.2 15.3 18.0 3.0 0.3 13.6 25.3 61.1 11 4.8 6.4 9.0 14.6 16.6 17.7 3.1 0.4 8.0 19.4 72.6 32 7.7 7.1 8.6 15.2 18.4 18.7 3.9 0.4 2.8 17.1 80.1 From comparison with added n-C22"marker", when ratio of coal to ketone used was 55, 3% of the palmitone remained unaltered and 3% as the n-C~6acid. Table 6. Major products from hydrous pyrolysis of di-n-alkyl ether at 330°C Duration of Ratio heating n-Alkanes (%) coal:ether (days) Clo Cii Ci2 CI3 Ci4 Ci5 CI6 0 3 1.6 5.7 15.4 5 3 0.4 0.6 1.0 2.5 4.5 8.8 29.0 2.5 5.5 - - 1.2 1.9 4.3 7.7 16.9 45.8 From comparison with the added n-C;2 "marker", when the ratio of coal to (Fig. 3). U n d e r conditions in which the ratio of added coal to ether was 50, the complexity of the reaction products prohibited a simple assessment of the product chromatograms. However, the virtual disappearance o f the ethers relative to the added n-C22 alkane " m a r k e r " demonstrated the complete decomposition of the ether under these conditions. Phenyl stearate. When pyrolysed alone, this ester was completely converted to the normal hydrolysis products, phenol and octadecanoic acid, together with 3.4% of the n-Cl7 alkane. When coal was added to the ester in the ratio 16:1 the hydrolysis products comprised almost 70% n-alkanes with the following distribution, C~2, 1.5%; Cm3, 3.7%; C~4, 5.4%; Cm5 10.1%; Clt, 8.8%; C17, 35.2% and Cls, 2.3%. The other products were phenol and octadecanoic acid.
Ci7 CIg
n-C~6 alcohol
Ether
Conversion of ether to n-alkancs
(%)
(%)
(%)
2.0 4.1 -71.0 29.0 3.7 3.6 8.7 37.2 54.1 4.3 6.5 3.7 7.6 88.7 ether used was 50, < I% of the ether remained unchanged.
Stearyl benzoate. In the absence of coal this ester was completely converted to a wide range of products, including benzoic acid, 11.5%; benzene, 13.3% and the n-C17 alkane, 3.3%. The major product was an unresolved mixture of the CI7 and Cms alkenes together with a small proportion of the n-C~8 alkane. When coal was added to this ester in the ratio o f 50:1, apart from benzene (14.4%) the products o f hydrous pyrolysis were n-alkanes with the following distribution Cl0, 1.5%; Cll, 1.3%; Ci2, 3.3%; Cj3, 6.0%; Cl(, 6.1%; Cls, 9.3%; Clt, 9.4%; ClT, 16.2% and Cls, 32.4%. n-Octadecyl phenyl ether. As shown in Table 7 the extent of conversion of the ether to n-alkanes was strongly dependent on the ratio o f added coal to ether. U n d e r all conditions used, n-octadecane was
"r 0
I o (.3 i
.1o QO
o
L
Produc~n- al kanes
Residual di-_n -ether
Fig. 3. (3as chromatogram of products from hydrous pyrolysis of the di-n-ether in the presence of coal (coal:ether ratio by weight = 5) at 330°C for 3 days.
J.W. SMITHet al.
370
Table 7. Major products from hydrous pyrolysis of n-octadecyl phenyl ether at 330°C for 3 days n-Alkanes (%) Ratio coal:ketone 2 6 20 53
CI4
Cts
CI~
Ci7
0.5 0.5
1.0 1.9
1.1 2.1
--3.4 4.3 6.3 10.9
Cis 5.0 9.5 14.8 65.6
the major alkane generated. In addition to phenol, n-octadecanol and CIs alkenes were significant reaction products, particularly when coal:ether ratios were small. Even when this ratio was increased to 53, the persistence of 9.2% of this ether in the reaction products testifies to the greater stability of the n-alkyl aryl ether relative to its di-n-alkyl counterpart. n-Hexadecyl phenyl ketone. This compound was very stable. In the absence of coal no reaction occurred. When a ratio of coal to ketone of 9 was used, 12% of the ketone was directly reduced to bexadecyl benzene and conversion to this compound was increased to 39% when a coal to ketone ratio of 52 was employed. Hydrocarbons (including n-octadecyl benzene, noctadecyl cyclohexane, and branched octadecyl isomers of benzene, cyclohexane and toluene). In the absence of coal and in reactions in which the ratio of added coal to model compound ranged from 5 to 98, no products indicating the cleavage of the side chain from the parent molecule were observed. Alkyl benzenes were reduced to the corresponding cyclohexane derivatives but the substituted toluenes remained unaffected.
Hydrous pyrolysis at 350°C with addition of coal n-Hexadecanoic acid. A ratio of added coal to n-acid of 15 was used in a single experiment at this temperature. Under these conditions 15% of the acid was converted to n-alkanes, a result not significantly different from that obtained by hydrous pyrolysis at 330°C (Fig. 1, Table 3). n-Octadecyl benzene. In another single experiment with an added coal to model compound ratio of 32, only 4% of the n-alkyl benzene was cleaved to generate mainly n-heptadecane with a little bexadecane. Toluene was also observed in the reaction products. Hydrous pyrolysis at 330°C with addition of molecular hydrogen n-Octadecanol. The reaction mixture, with a ratio of added coal to the n-alcohol of 4, was pressurized to 6.9-7.4 MPa with molecular hydrogen before closure of the vessel and heating to 330°C. The reaction products consisted of octadecane, 11.5%; heptadecane, 0.6%; hexadecane, 0.3%; mixed octadecenes, 2.1%; and unchanged alcohol 85.6%. Phenyl stearate. Hydrous pyrolysis of this ester in the absence of coal at a pressure of 6.9-7.4 MPa of molecular hydrogen resulted in complete hydrolysis
Alkenes Cis (%) 6.5 11.0 5.9 0.0
Alcohols n-Cls (%)
Phenol (%)
Ether (%)
2.8 4.7 3.9 4.0
8.0 7.8 13.0 10.3
74.3 60.2 51.6 9.2
to yield octadecanoic acid and phenol. No n-alkanes were observed in the reaction products.
Experiments with miscellaneous additives n-Hexadecanoic acid. In attempts to promote decarboxylation of the acid or direct reduction to the n-alkane, a range of additives was tested, including alumina, calcium hydroxide, copper chromite, basic copper carbonate, zinc and tin powders and various combinations of these additives together with coal. Temperatures in the range 275-330°C were used. No significant increases in n-alkane generation compared to those obtained by additions of coal alone were observed. n-Octadecanol. Similar attempts to improve the conversion of the alcohol to alkanes by the use of the additives listed above were also unsuccessful. DISCUSSION
From the data presented above on the hydrous pyrolysis of model compounds it is evident that the degree of conversion of n-alkyl components to nalkanes is dependent on (1) the chemical mode of linkage of the n-alkyl groups in the parent model compound and (2) the ratio of coal to model compounds employed in hydrous pyrolysis.
Relationship between chemical mode of attachment of n-alkyl components and ease of conversion to nalkanes To simplify discussion, the model compounds have been classified into four groups based on the ease with which conversion to n-alkanes was obtained. These groups--complete, moderate, difficult, resist a n t - a r e defined in Table 8 on the basis of the degree of conversion measured, or to be expected, when a ratio of coal to model compound of ,,, 80 is used in hydrous pyrolysis. As mentioned previously, at such large ratios of coal to model compound, a real difficulty exists in distinguishing the precise origins of the shorter chain n-alkanes in reaction products and in determining the extent of chain-shortening reactions. Therefore values given for conversion under such conditions may only be regarded as guides to the actual situation. Two factors related to molecular structure appear to be critical in determining the degree of conversion undergone by the model compounds studied: (1) the inherent thermal stability of the molecule and (2) the tendency for the molecule to form more stable prod-
Hydrous pyrolysis of model compounds ucts by cleavage or rearrangement under thermal stress. The model compounds classified as "complete" in Table 8 show none of these adverse characteristics. The n-alcohol, when sufficient coal is present, appears to transform via the alkene and the aldehyde to the corresponding n-alkane. Therefore the formation of small quantities of n-alcohols during the hydrous pyrolysis of the di-n-ether and the n-alkyl aryl ether is not regarded as a barrier to their total conversion. Similarly the alcoholic n-alkyl component of the n-octadecyl benzoate is rapidly freed by hydrous pyrolysis and proceeds along the same alkene pathway as the product n-alkanes. Benzoic acid, the other hydrolysis product, underwent partial decarboxylation to benzene. On precisely the same type of evidence, the di-nester was classified as moderate. In this case, following hydrolysis, the alcoholic portion of the ester was again free for conversion to n-alkanes, but the carboxylic acid fraction, now comprising some 50% of the total n-alkyl content of the original ester, was particularly resistant to further conversion by decarboxylation (Tables 3 and 4). On hydrous pyrolysis of the di-n-pentadecyl ketone, hexadecanoic acid was formed to a small degree by rearrangement of the parent molecule (Table 5). Therefore this ketone is classified as "moderate" by the criteria applied. Carboxylic acids and aryi esters of n-acids both automatically qualify as "difficult" in Table 8 because in esters of this type the total n-alkyl content is released as the n-acid on hydrous pyrolysis. Acids are partially resistant to decarboxylation, the major pathway for n-alkane formation, as shown in Table 3 and Fig. 1. The stability of acids under these conditions is in general agreement with the findings of Jurg and Eisma (1964), Kawamura and Ishiwatari (1985) and Kawamura et al. (1986a, b) discussed earlier. n-Alkyl aromatic and hydroaromatic compounds, and those compounds which are converted to such structures on hydrous pyrolysis, e.g. n-alkyl aryl ketones, were totally resistant to reaction. Clearly carbon-carbon bond cleavage in parent molecules of the types examined is not accomplished by hydrous
371
pyrolysis; n-alkanes added as "markers" exhibit a similar thermal stability. Nevertheless, the data presented in Tables 2-7 demonstrate unequivocally that when n-alkanes are generated by other mechanisms, e.g. by decarboxylation or by dehydration and alkene formation, significant cleavage of the generated carbon chains into smaller units occurs. Mechanisms relating to n-alkane generating processes in hydrous pyrolysis have been described by Hoering (1984).
The role of brown coal in promoting hydrous pyrolytic reactions From earlier work (Brooks and Smith, 1969; Hoering, 1984) and the bulk of the experimental data reported here, it is evident that the presence of carbonaceous material is essential if the generation of n-alkanes from the n-alkyl components of organic molecules is to proceed to a significant extent via hydrous pyrolysis in laboratory experiments and presumably in sedimentary environments. This role cannot be adequately described as catalytic since the extent of model compound conversion is closely related to the quantity of the brown coal added and, to achieve significant conversion, ratios of coal to model compounds >25 are commonly required. Moreover, from the data presented here, it is evident that the brown coal fills several roles. For example from Table 3, the generation of n-octadecane as the major hydrous pyrolysis product of n-octadecanol and the disappearance of the unsaturated Cls hydrocarbons in these products as the quantity of coal added is increased, suggests that a major role of the coal is as a hydrogen donor. On the other hand it is obvious from Table 4 that generation of n-alkanes from n-acids proceeds via a decarboxylation process promoted by the presence of the active surface of coal. Information on the nature of the hydrogeneration mechanisms prevailing during hydrous pyrolysis was provided by those experiments using n-octadecanol and phenyl stearate in which the autoclave was pressurized with molecular hydrogen to 6.9-7.4 MPa prior to hydrous pyrolysis. When using a coal to model compound ratio of 4:1, 27.9% of the n-
Table 8. Relative convertibilities of n-alkyl containing model compounds to n-alkanes by hydrous pyrolysis
Classification Complete
Moderate Difficult Resistant
Model compounds n-Alcohols Di-n-ethers n-Alkyl aryl ethers n-Esters of aryl acids Di-n-esters Di-n-ketones n -Acids Aryl esters of n-acids Aryl-alkyl ketone n-Alkyl aromatics and hydroaromatics
Conversion of of n-alkyl component to n-alkanes (%)
n-Alkyl compounds resistant to conversion to n-alkanes
> 90% > 70%
Minor n-acids
< 70% < 70% 0 0
Parent n-acid Major n-acids n-Alkyl aromatic Parent hydrocarbons and hydroaromatic derivative
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J.W. SMITHet al.
octadecanol was converted to n-alkanes by normal hydrous pyrolysis (Table 2), however, when the system was pressurized with molecular hydrogen as described, this conversion was reduced to 13.6%. Rather than promoting reduction, the molecular hydrogen appeared to behave as an inert diluent gas. A similar effect was seen when molecular hydrogen was introduced into experiments in which phenyl stearate underwent hydrous pyrolysis in the absence of the coal. In these experiments, whether hydrogen was added or not, complete hydrolysis of the ester to phenol and octadecanoic acid occurred. However, in the absence of added hydrogen, the reaction products also contained 3.40 heptadecane. Where the reactor had been pressurized with hydrogen, n-alkanes were not detected in the products. On this evidence, one role of the brown coal appears to be as a source of exchangeable active hydrogen, although the availability of such species for the reduction of model compounds is relatively small, in view of the high ratio of coal to model compound required to effect reduction. The role of the brown coal in promoting decarboxylation remains obscure. The quantity and composition of the inorganic and mineral matter associated with the brown coal and the ineffectiveness of additions of a range of inorganic materials in increasing decarboxylation provide little support for an inorganic decarboxylation mechanism. Another unknown factor in hydrous pyrolysis is the effect of the nature and composition of the carbonaceous material on conversion reactions. To date, although the hydrous pyrolysis products from several shales and coals have been analysed and evaluated (Brooks and Smith, 1969; Winters et al., 1983), only two materials, the medium-light brown coal lithotype used in the present study and a solvent extracted Messel shale (Hoering, 1984), have been used in experiments specifically designed to define the role of the carbonaceous additive in hydrous pyrolysis. It does not seem unlikely that the capacity of the carbonaceous material to generate active, reductive species, and the composition of associated mineral matter play a significant role in determining the degree of conversion obtained and the nature of the reaction products. Use o f hydrous pyrolysis in assessment o f long-chain n-alkyl contents o f sediments
The high degree of dependence of hydrous pyrolytic reactions on the presence and concentration of carbonaceous materials, the difficulties encountered in converting n-acids to n-alkanes and the complete resistance of n-alkyl aromatic and hydroaromatic hydrocarbons to any change suggest that caution should be observed in the assessment of results. From the evidence present, the results obtained for nalkane contents and distributions in such rocks are likely to be most in doubt in sediments in which (a) free n-carboxylic acids or their metal salts occur,
(b) carboxylic acids are released on hydrolysis and (c) n-alkyl aromatic and hydroaromatic compounds constitute a major portion of the total n-alkyl content. Monthioux et al. (1985) reported that many aspects of the behaviour of Type III organic matter during natural evolution could be reproduced satisfactorily by pyrolysis in a confined system. Moreover, hydrous pyrolysis appears to simulate sedimentary maturational processes when hydrocarbon products are compared with related crudes (Brooks and Smith, 1969; Winters et al., 1983). However difficulties arise when attempts are made to relate the changes in vitrinite reflectance that accompany hydrous pyrolysis to specific chemical reactions. For example, it appears to be well established within the coal rank series that the content of carboxylic groups in coals with carbon contents greater than 80% (Blom, 1960) or with vitrinite reflectance values in excess of 0.7*/0 (Evans et al., 1984) is negligible. However, in experiments reported here, at temperatures of 350°C, decarboxylation of hexadecanoic acid only proceeded to 15% although the vitrinite reflectance of the brown coal included in the reactants had increased from 0.2 to 1.3%. CONCLUSIONS Although the product oils from hydrous pyrolysis may in many respects resemble crudes, the data presented here indicate that the conversion to hydrocarbons of some important n-alkane precursors may be incomplete. Factors that require consideration in the assessment of hydrous pyrolysis as a method for the determination of the long-chain n-alkyl contents of source rocks and hence as a guide to their petroleum potential include: (a) the stability of n-alkanoic acids and nalkylated hydrocarbons under the hydrous pyrolytic conditions investigated (b) the major, but little-understood, role of brown coal (kerogen) in promoting n-alkane generation and release; (c) possible variations in this role with kerogen type; and (d) difficulties in relating specific chemical changes observed with increases in vitrinite reflectance. Acknowledgements---Grateful acknowledgments are made to the State Electricity Commission of Victoria for provision and analysis of the brown coal. REFERENCES
Blom L. (1960) Analytical Methods in Coal Chemistry. Luijk, Eindhoven. Brooks J. D. and Smith J. W. (1969) The diagensis of plant lipids during the formation of coal, petroleum and natural gas--II. Coalification and the formation of oil and gas in
Hydrous pyrolysis of model compounds the Gippsland Basin. Geochim. Cosmochim. Acta 33, 1183-1194. Evans E. J., Batts B. D. and Smith J. W. (1984) Determination of the hydrocarbon prospoaivity of sediments by hydrogenation. APEA J. 24, 222-229. Hoofing T. C. (1984) Thermal reactions of kerogen with added water, heavy water and pure organic substances. Org. Geochem. S, 267-278. Jurg J. W. and Eisma E. (1964) Petroleum hydrocarbons: generation from fatty acid. Science, N.Y. 144, 1451-1452. Kawamura K. and Ishiwatari R. (1985) B¢haviour of lipid compounds on laboratory heating of a recent sediment. Geochem. J. 19, 113-126. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986a) Volatile organic acids generated from kerogen during laboratory heating. Geochem. J. 20, 51-59. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986b) Long-chain carboxylic acids in pyrolysates of Green River kerogen. Org. Geochem. 10, 1059-1065. Lcwan M. D. (1985) Evaluation of petroleum generation by hydrous pyrolysis experimentation. Phil. Trans. R. Soc. Lond..4,315, 123-134. Lewan M. D., Winters J. C. and McDonald J. H. (1979) Generation of oil-like pyrolyzates from organic-rich shales. Science, N.Y. 203, 897-899.
OG 14/~--B
373
Monthioux M., Landals P. and Monin J-C. (1985) Comparison between natural and artificial maturation series of humic coals from the Mahakam delta, Indonesia. Org. Geochem. 8, 275-292. Price L. C. and Barker C. E. (1985) Suppression of vitrinit¢ r¢fl¢ctanc¢ in amorphous rich kerogon--a major unrecognized problem. J. Pet. Geol. 8, 59-84. Rigby D., Gilbert T. D., Batts B. D. and Smith J. W. (1986a) The generation and release of hydrocarbons from Victorian brown coal lithotypes. In Second South-Eastern Australia Oil Exploration Symposium, Technical Papers Presented at PESA Symposium, 14-15 November 1985, Melbourne (Edited by Glenie R. C.), pp. 433-438. Pet. Explor. Soc. Australia, Melbourne. Righy D., Gilbert T. D. and Smith J. W. (1986b) The synthesis of alkyl aromatic hydrocarbons and its geochemical implications. Org. Geochem. 9, 255-264. Smith J. W., Batts B. D. and Gilbert T. D. (1987) A quest for a new parameter in petroleum exploration geochemistry. APEA J. 27, 98-105. Winters J. C., Williams J. A. and Lewan M. D. (1983) A laboratory study of petroleum generation by hydrous pyrolysis. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 524-533. Wiley, New York.